The present invention relates to a secondary air supply system for the exhaust system of an internal combustion engine, and, more particularly, to an air control valve incorporated in the secondary air supply system.
In an exhaust gas purifying system which incorporates a three-way catalyst for simultaneously removing HC, CO, and NOx contained in the exhaust gases of an internal combustion engine, the air/fuel ratio of the exhaust gases must be controlled to be within a relatively narrow range of the stoichiometric air/fuel ratio in order to obtain effective performance of the three-way catalyst. Therefore, in the exhaust gas purifying system incorporating a three-way catalyst, the air/fuel ratio of engine intake mixture is set on the smaller or rich side of the stoichiometric air/fuel ratio, and the exhaust gases generated from such a mixture are supplied with secondary air while the air/fuel ratio is monitored by an oxygen detector so that the air/fuel ratio of the exhaust gases introduced into the three-way catalyst is maintained within a relatively narrow range (the window range) around the stoichiometric air/fuel ratio which is required to obtain effective performance of the three-way catalyst.
A secondary air supply system which supplies secondary air to the exhaust system of an engine for the aforementioned purpose generally comprises a source of compressed air such as an air pump driven by the engine, an air control valve which supplies a part of the air delivered from said source to the exhaust system of the engine while relieving the rest of the air, an oxygen detector for detecting residual oxygen contained in the exhaust gases flowing through the exhaust system, a source of actuating fluid pressure (for which the intake manifold generally serves to supply intake manifold vacuum as the actuating fluid pressure), a change-over valve for said actuating fluid pressure, and a controller which changes over said change-over valve in accordance with the output of said oxygen detector, said air control valve supplying the air delivered from said source of compressed air to the exhaust system as secondary air when said oxygen detector detects no residual oxygen while it stops supplying secondary air to the exhaust system while relieving the air supplied from said source of compressed air to the atmosphere, or, generally, into the air cleaner of the engine, when the oxygen detector detects residual oxygen. The air control valve incorporated in the conventional secondary air supply system generally comprises an inlet port for receiving air from a source of compressed air such as an air pump driven by the engine, an outlet port for supplying a part of the air received to the exhaust system, a relief port for relieving the rest of the air received, a first passage which connects said inlet port and said outlet port, a second passage which connects said inlet port and said relief port, a valve element which reciprocally controls the openings of said first and second passages, first and second diaphragm chambers selectively supplied with either intake manifold vacuum or atmospheric pressure by way of said change-over valve, and at least one diaphragm which defines said individual diaphragm chambers and is connected with said valve element, wherein said diaphragm is adapted so as to shift said valve element in the direction to open said first passage and to close said second passage when said first diaphragm chamber is supplied with intake manifold vacuum while said second diaphragm chamber is opened to the atmosphere, and so as to shift said valve element in the direction to open said second passage and to close said first passage when said second diaphragm chamber is supplied with intake manifold vacuum while said first diaphragm chamber is opened to the atmosphere.
The secondary air supply system for the exhaust system of an internal combustion engine which incorporates an air control valve of the aforementioned structure together with an oxygen detector, a vacuum change-over valve, and a controller which changes over said vacuum change-over valve in accordance with the output of said oxygen detector is a feedback control system which supplies additional air as the secondary air to the basic exhaust gases having an air/fuel ratio which is somewhat lower than the lower limit of the window range, whereby the air/fuel ratio of exhaust gases is controlled in a manner such that it changes in the shape of triangular pulse waves going up and down on either side of the center of the window range. The responsiveness of this feedback control system is relatively low, as in the order of a few Hertz at the greatest. Therefore, when the system is designed as a compromise between quickness of response to transient conditions and accuracy, the air/fuel ratio generally overshoots and undershoots out of the window region; i.e., the air control valve is periodically fully opened or fully closed. In such a secondary air supply system, when the mean level of basic air/fuel ratio changes from the standard level due to changes of operational condition of the engine or allowable errors in products, the difference between stoichiometric air/fuel ratio and basic air/fuel ratio changes, whereby the extent of overshooting or undershooting of air/fuel ratio relative to stoichiometric air/fuel ratio changes. As a result, the mean air/fuel ratio of exhaust gases deviates from the stoichiometric air/fuel ratio, and the accuracy of air/fuel control lowers as a whole, thereby reducing the effectiveness of the three-way catalyst. Furthermore, when the three-way catalyst has been used for such a long time that its effectiveness has lowered and the width of its window region has been reduced, this overshooting & undershooting will bring the air/fuel ratio further away from the window region, thereby further reducing the effectiveness of the three-way catalyst.